Python安装：Mac 使用brew 安装Python2 和 Python3

1. 安装python

   ## python2
   brew install python  
   
   ## python3
   brew install python3
   

2. 出现错误

   Error: An unexpected error occurred during the `brew link` step
   The formula built, but is not symlinked into /usr/local
   Permission denied @ dir_s_mkdir - /usr/local/Frameworks
   Error: Permission denied @ dir_s_mkdir - /usr/local/Frameworks
   

3. 解决

   手动创建目录

   $ sudo mkdir /usr/local/Frameworks
   $ sudo chown $(whoami):admin /usr/local/Frameworks
   

4. 查看版本

   $ which python
   /usr/bin/python
   
   $ python --version
   Python 2.7.16
   

5. 设置环境变量

   ## 打开配置文件，如果没有就在~目录下面创建一个
   $ open ~/.bash.profile
   

   设置python环境变量路径

   # Setting PATH for Python 2.7 系统自带python2
   PATH="/System/Library/Frameworks/Python.framework/Versions/2.7/bin:${PATH}"  -
   export PATH
   
   # Setting PATH for Python 3.7.5 brew安装的python3
   PATH="/usr/local/Cellar/python/3.7.5/bin:${PATH}"
   
   ## 设置别名 注意目录看下是否正确
   alias python2='/System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7'
   alias python3='/usr/local/Cellar/python/3.7.5/bin/python3'
   # alias python=python3
   

   生效配置文件

   source ~/.bash.profile 
   

6. 验证

   ## python 和 python2 都指向python2
   $ python --version
   $ python2 --version
   $ python3 --version